Lead zirconate titanate, which contains lead, is a representative piezoelectric material and has been used in various piezoelectric devices, such as an actuator, an oscillator, a sensor, and a filter. However, it has been pointed out that when a lead-containing piezoelectric device is once discarded and exposed to acid rain, a lead component contained in a piezoelectric material is dissolved therein and absorbed in the soil, and thereby the ecological system is probably adversely influenced. Hence, in order to realize a lead-free piezoelectric device, research and development of lead-free piezoelectric materials has been energetically carried out.
According to NPL 1, it was found that when a small amount of barium titanate is solid-solved in sodium niobate, which is an antiferroelectric substance, sodium niobate is changed into a ferroelectric substance. Furthermore, NPL 1 has disclosed the remanent polarization, coercive field, piezoelectric constant, and electromechanical coupling coefficient, which are obtained when a composition containing barium titanate at a concentration of 5% to 20% is sintered at 1,200° C. to 1,280° C. A lead-free material disclosed in NPL 1 also contains no potassium which makes sintering difficult and degrades humidity resistance. Furthermore, the Curie temperature of the material disclosed in NPL 1 is higher than the Curie temperature (110° C. to 120° C.) of barium titanate, which is a representative lead-free piezoelectric material. NPL 1 also has disclosed that the Curie temperature of a composition (Na0.9Ba0.1)(Nb0.9Ti0.1)O3 at which a maximum piezoelectric constant d33 of 143 pC/N can be obtained is 230° C.
In addition, PTL 1 has disclosed that when cobalt is added to a piezoelectric ceramic which is a solid solution of sodium niobate and barium titanate, the piezoelectric constant is improved. In addition, PTL 1 has also disclosed that among the piezoelectric materials of PTL 1, at least one sample is included which is difficult to be polarized due to a low insulating property of 106Ω or less.